Cobalt-catalyzed selective dehydrocoupling polymerization of prochiral silanes and diols

https://doi.org/10.1016/j.eurpolymj.2020.109832Get rights and content

Highlights

  • Cobalt-catalyzed selective dehydrocoupling polymerization of prochiral silanes and diols.

  • Novel PSEs with pendant Sisingle bondH groups were obtained from different monomers.

  • Co-catalyzed one-pot two-step process was also conducted.

  • Asymmetric version of selective dehydrocoupling polymerization was realized.

Abstract

As a kind of material possessing thermal stability, biocompatibility and degradability, polysilylethers (PSEs) have potential applications. Different catalytic systems have been developed for the synthesis of PSEs. However, the resulted PSEs have similar structure, which is unfavorable for adjusting its properties. We herein synthesized a series of novel PSEs with pendant Sisingle bondH groups via cobalt-catalyzed selective dehydrocoupling polymerization. Various diols and prochiral silanes are suitable for this method. This novel PSEs could also be obtained via a Co-catalyzed one-pot two-step process. Furthermore, asymmetric dehydrocoupling polymerization was studied and high-Mn PSE was obtained. After the cleavage of PSE by methyl magnesium iodide, chiral silane was obtained with high yield and moderate enantioselectivity.

Graphical abstract

A cobalt-catalyzed selective dehydrocoupling polymerization was presented, resulting in novel PSEs with pendant Sisingle bondH groups. Different types of monomers are suitable substrates for the polymerization. A series of PSEs were provided with good yields and high molecular weight. Asymmetric dehydrocoupling polymerization was also realized by employing the chiral bisphosphine ligand, giving the chiral PSE with 65% ee.

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Introduction

Because of possessing various properties and abundant reserves of Si and O in the earth’s crust [1], developing polymer containing a silicon-oxygen bond in the mainchain has been a hot topic. Until now, several polymeric materials containing Sisingle bondO bond in the mainchain have been developed and applied, including polysiloxanes, polysilylethers, polysilylesters, etc. [2]. These polymeric materials usually exhibit similar properties, such as low Tg, good thermal stability, biocompatibility, high gas permeability, degradability, etc. [3]. Among these polymers, polydimethylsiloxane (PDMS) possesses high flexibility and thermal stability, which has been successfully used as silicon oil, elastomer, adhesives, coatings, etc. [4], [5], [6].

Due to the combination of favorable properties of polycarbosilanes and polysiloxanes, polysilylethers have gradually attracted the attention in the past few decades [2]. In the early work, the synthesis of PSEs was often realized by the polycondensation of diols and dichlorosilanes or dianilinosilanes [7]. However, equivalent base or vacuum condition is required to furnish high-Mn PSEs, which has negative influence on the atom-economy and further application of polymerization [8]. Afterwards, Nishikubo’s group synthesized a new kind of polysilylethers with reactive pendant chloromethyl groups via TPBC catalyzed polyaddition of bis(epoxide)s and volatile dichlorosilanes [9]. In order to replace the unstable chlorosilanes, silanes were gradually applied to the synthesis of PSEs via dehydrocoupling [10] and hydrosilylation polymerization [11].

As an efficient and high atom-economic way to synthesize PSEs, dehydrocoupling polymerization has been catalyzed by the catalysts derived from precious transition metals such as palladium, platinum, rhodium, and ruthenium [12]. Various silanes and diols were used for providing PSEs with different thermal stability and degradability. In despite of these advance, drawbacks of these catalytic systems based on noble metals, such as high cost, low abundance, high catalyst loading and biological compatibility become increasingly the key problem of its development [13]. Thus, use of catalysts based on first-row transition metals like iron (Fe), manganese (Mn) and cobalt (Co) is much more sustainable for addressing the disadvantages of precious metals. Recently, the catalytic systems derived from manganese and iron have been reported [14]. However, Co-catalyzed dehydrocoupling polymerization of silanes and diols has not been developed. Moreover, another problem of the development of PSEs is that most of the PSEs reported before have similar structure and framework, which is unfavorable for adjusting its properties and expanding its applications. In 2000, Rh-catalyzed selective dehydrocoupling of prochiral silanes and diols was developed by the group of Kawakami, giving novel PSEs with pendant Sisingle bondH groups and up to 39.8% average ee of silicon atoms [15]. The reactive Sisingle bondH groups of these PSEs may react with water or divinyl compounds, which will have potential in the application as chiral column packing materials. Although the stereoselectivity of the polymerization was still not satisfactory, these results provided a new developing trend of PSEs. In the context of our continuous interest in transition-metal-catalyzed dehydrocoupling polymerization [16], herein we reported cobalt-catalyzed selective dehydrocoupling polymerization of prochiral silanes and diols, furnishing a series of high-Mn PSEs containing pendant Si-H groups. Furthermore, an asymmetric version of this transformation was also studied; moderate 65% of enantioselectivity was obtained. (Scheme 1).

Section snippets

Results and discussion

Original experiments employed silane 1a and diol 2a at 60 °C in the presence of Co(acac)2/dppb as a catalyst (Table 1). Initially, the reaction was conducted in THF, giving a polymer with moderate Mn and high yield (entry 1). When the ligand was removed, an oligomer was obtained, which indicated phosphine ligand play a vital role in the polymerization (entry 2). Then, the solvent effect was screened. It was found that high yields were also achieved in MeCN, tBuOMe and nhexane (entries 3,5,7).

Conclusions

In summary, we have developed Co-catalyzed selective dehydrocoupling of prochiral silanes and diols with a catalyst generated from Co(acac)2 and dpppe. Different types (AB type or AA and BB type) of monomers are suitable substrates for the polymerization. A series of novel PSEs with pendant Si-H groups were provided with good yields (up to 89% yield) and high molecular weight (up to 32.3 kg/mol). Moreover, the Co-catalyzed one-pot two-step process was realized. An asymmetric version of

Reagents and instrumentation

All reactions were carried out under an atmosphere of nitrogen using the standard Schlenk techniques, unless otherwise noted. Diols 2a-f were recrystalized from ethyl acetate. Other commercially available reagents were used without further purification. Solvents were treated prior to use according to the standard methods. 1H NMR and 13C NMR spectra were recorded at room temperature in CDCl3 on 400 MHz instrument with tetramethylsilane (TMS) as internal standard. Flash column chromatography was

CRediT authorship contribution statement

Xiao-Yong Zhai: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft. Xiao-Qing Wang: Investigation. Yong-Gui Zhou: Supervision, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

Financial support from National Natural Science Foundation of China (21690074), Dalian Institute of Chemical Physics (UN201701) and Chinese Academy of Sciences (XDB17020300) is acknowledged.

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